Field of the Invention
The invention relates to a temperature-protected electrical switch component, and in particular to a semiconductor switch. A first temperature sensor is provided at a first location. The first sensor causes the switch component to switch a load current off when the temperature detected by the sensor exceeds a first threshold value and to switch the load current on again when the temperature detected by the temperature sensor falls below a second threshold value, which is lower than the first threshold value. The load current thus forms an oscillation. A temperature-protected electrical switch component of this type is, for example, a temperature-protected field-effect transistor (PROFET, HITFET, TEMPFET).
Temperature-protected field-effect transistors, which are mainly used as semiconductor switches, are supposed to switch off a load current in the event of a short circuit as soon as the temperature at the field-effect transistor reaches a specific upper limit. Thus, printed circuit boards in certain housings can only withstand temperatures of up to about 150.degree. C., because the soldered points of these printed circuit boards are destroyed at higher temperatures. In other words, when the temperature of 150.degree. C. is exceeded, a temperature sensor and a logic circuit assigned to the sensor are supposed to ascertain a short-circuit situation and switch off the load current by means of the field-effect transistor that is operated as a semiconductor switch.
The diagram of FIG. 5 illustrates a prior art temperature-protected field-effect transistor 1 of this type. An operating voltage of 25 V and an input voltage of 5 V, for example, are applied to the temperature-protected field-effect transistor 1. When the temperature exceeds approximately 150.degree. C., then a load current I is switched off.
In FIG. 6 the temperature-protected field-effect transistor of FIG. 5 is illustrated in a diagrammatic sectional view. A metal plate 2 carries a semiconductor body or chip 3 with an integrated circuit part (IC part) 4 and a field-effect transistor 5. A source and a drain of the transistor 5 are illustrated diagrammatically. A temperature sensor 6 is disposed in the region of the cell array of the field-effect transistor 5. The temperature sensor 6 may be a diode or a bipolar transistor.
Referring now to FIG. 7, there is shown the electrical circuit configuration of the temperature-protected field-effect transistor 1. The temperature sensor 6 lies in the region of the field-effect transistor 5, through which the load current I flows. The IC part 4 forms a logic circuit that can be used to set an upper temperature threshold and a lower temperature threshold. When the temperature measured by the temperature sensor 6 exceeds the upper threshold value, then the logic circuit switches the field-effect transistor 5 off. The load current I stops to flow. As a result, the region around the temperature sensor 6 begins to cool down. When the temperature measured by the temperature sensor 6 then reaches the lower threshold value, then the logic arrangement switches the field-effect transistor 5 back on. The load current I begins to flow again. Once the upper temperature threshold has been reached once more, as measured by the temperature sensor 6, the field-effect transistor 5 is switched off again until the lower temperature threshold is reached.
This results in a periodic switch-on/switch-off operation, as graphed in FIG. 8. The temperature of the semiconductor body 3 settles to a value between the two threshold values. The period time in this case depends on the cooling, in particular via the metal plate 2.
Temperature-protected semiconductor switches are particularly used together with lamps and motors as loads. It has been shown, however, that these loads do not switch back on after a short circuit because the surge current that occurs on reconnection is sensed as another short circuit.